Update Category.java

Added missing method from log4j Category class, which is quite useful and can be easily implemented.
(Found it when using library which called this method so I received NoSuchMethodError)

log4j-over-slf4j/src/main/java/org/apache/log4j/Category.java

@@ -350,5 +350,10 @@ public class Category {
     public boolean getAdditivity() {
         return false;
     }
+    
+    public void assertLog(boolean assertion, String msg) {
+        if(!assertion)
+            error(msg);
+    }
 
 }